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Q I have a 1998 Nissan Sentra with a 1.6-liter engine. It does not leak or use any oil except at highway speeds when it uses up to a quart of oil on a 300-mile trip. I replaced the PCV valve and it seemed like the problem was fixed as there was no oil usage for a trip or two. But now it is acting up again. Is it possible the new PCV valve is at fault? It seems to be working properly and, when I remove it at engine idle, it makes a hissing sound and there is a strong vacuum when I place my finger over the valve inlet. The ventilation hoses seem fine and unobstructed. Any ideas?

A Ah, such fond memories. My old Jeep Grand Wagoneers used to suffer this problem. The engines were "tired" -- lots of miles on them on the big V-8's - so the excess combustion blow-by into the crankcase overwhelmed the PCV system's capability to draw these vapors into the induction system and burn them. The excess crankcase pressure also forced oil past the piston rings as well as into the PCV system, progressively choking it off and causing high oil consumption. Back then I had a simple fix: Replace the PCV valve every six months and "roto-rooter" out the PCV passages with part of an old speedometer cable.

This may well be the issue with your Nissan. Make sure the crankcase is properly sealed at the oil dipstick, oil filler opening, valve cover, etc. Any air leaks into the crankcase add to the burden of the PCV system. Clean out the PCV ports and passages as best you can, install another new PCV valve and recheck the oil consumption.

Another cause of freeway oil consumption is excessively high oil operating temperatures. Synthetic oils are more stable at high temperatures and may reduce consumption.

Q The turn signals on my 2004 Ford Explorer have started working intermittently. Sometimes they work perfectly. Other times they work and then the small inside arrow stops blinking and so does the outside turn light. And sometimes just the inside little arrow light goes on and there's no blinking at all. I've checked the fuse under the dashboard. I've made sure the yellow module is still attached. Any suggestions?

A Although the multi-function/turn signal switch or its connector in the steering column may be at fault, I'd start with the more accessible components. First, check the function of the four-way/emergency flashers -- do they work? If not, the flasher/indicator relay C2047 may be at fault. It is located in the central junction box under the dash to the left of the driver. The tricky part is that the relay panel in this box is on the back side of the fuse panel, meaning the panel must be partially disassembled to gain access to the relays.

And as always with electrical gremlins, a poor connection or ground at the turn signal lamp could cause this intermittent operation. Also, pin #5 of the relay is ground.

Q I have a 1997 Nissan Altima with more than 220,000 miles on it. It starts right away, but when warmed up and driving, it jerks and hesitates as I come to a stop, then stalls. The car starts right back. I had the car towed to a shop. The pigtail and ground wires were checked and the following parts changed: four new plugs, knock sensor and crankshaft sensor. I drove the car several times after that and the problem persists. So back to square one. Please help.

A The shop may be looking in the wrong direction. If the torque converter clutch solenoid valve is not fully releasing, the converter clutch may be staying engaged as the car is brought to a stop.The symptom is just like failing to push in the clutch pedal as a manual transmission vehicle is brought to a stop.